1. Field of the Invention
This invention relates generally to multiport tunable optical filters, for example as may be used for optical channel monitoring.
2. Description of the Related Art
Optical devices that can be tuned to select one or more wavelengths from a wider wavelength spectrum are known as tunable optical filters. They are used in a variety of optical systems, e.g., wavelength division multiplexed (WDM) systems. In WDM systems that operate typically over wavelength bands of tens of nanometers, tunable optical filters are used for optical performance monitoring (OPM) to ensure that signal power, signal wavelength, and/or optical signal to noise ratios (OSNR) are within specified limits. Other applications for tunable optical filters include optical noise filtering, noise suppression, the actual wavelength division demultiplexing function, and optical routing.
Dense wavelength division multiplexed (DWDM) systems have many wavelength channels operating across a wide optical spectrum. Additionally, many tunable optical filters are limited in terms of how their fiber input ports and fiber output ports are configured and the number of those ports. For example, a large linear array of ports may increase optical aberrations as the ports move away from the optical axis of the system, thus limiting the size of the array. Additionally, adding ports in this manner increases the form factor (e.g., size) of the device.
Additionally, with the introduction of DWDM systems operating on a flexible grid, a channel monitoring device may be required to report the power in a given frequency range, identified by a start and a stop frequency. Additionally, the system may be required to be able to locate these frequencies with an accuracy of 1 GHz. This is not readily done through calibration, as small changes in the optical path over the lifetime of the device can easily account for shifts that easily exceed 1 GHz. Moreover, the accuracy of 1 GHz is difficult to achieve even with a bench-top instrument.